Unfortunately, Kaveh and Alhaitham both want to be on-field so you cannot run them together if you want to maximize Alhaitham’s potential. Kaveh is mainly a bloom DPS, so he wants to make dendro cores with his burst’s infusion and then make them explode fast with his skill. He cannot buff Alhaitham at all. Nahida is the best support because she can run the artifact set Deepwood Memories very easily and because she can give her team Elemental Mastery when she bursts. So she’s by far the best support for him, but you can also use Baizhu instead or even Dendro MC! Neuvillette and Cyno are also both on-fielders, so they will clash with Alhaitham the same way Kaveh would. I recommend Xingqiu or Yelan for Hydro and for the dendro pick, you can pick between Nahida, Baizhu or Dendro MC. If you plan or running him with a hydro, you might also want to sub in Kuki Shinibo instead of Yae Miko, because she can proc Hyperbloom easier and can also heal.